As shown in FIG. 20 hereof, a gear 100 is a machine element having a plurality (ten in the illustrated embodiment) of teeth 101 at equal pitches. Pitch circle 102 is a main dimensional reference of the gear 100, and tooth thickness T is defined as a thickness of the tooth 101 measured at the pitch circle diameter. Tooth groove is defined as a groove between each pair of adjacent teeth 101, and the width W of the tooth groove is also one of main dimensions of the gear 100.
FIG. 21 hereof is an enlarged view of a part 21 shown in FIG. 20. As shown in FIG. 21, the gear 100 may occasionally have a dent or nick 103 on a surface of a tooth 101 thereof. The nick 103 is produced due to, for example, collision with another gear during transportation, or abutment with a jig or a tool during manufacture.
When the tooth 101 is subject to a local impact, a pit or recess is formed in a tooth surface while, at the same time, a peripheral edge of the recess rises from the tooth surface. Thus, the nick 103 is constituted by a recess 104 and a projection 105 along a peripheral edge of the recess 104. A problem will occur when the height of the projection 105 exceeds a predetermined level.
For instance, when the gear 100 having the nick 103 is used in combination with a mating gear (not shown), a large noise (abnormal noise) will occur because tooth flanks of the mating gear come into point contact with the projection 105 of the nicked gear 100, rather than surface contact which is normally established when the gear 100 has a smooth tooth flank 106 (indicated by phantom line shown in FIG. 21). To deal with this problem, the nicked tooth is burnished to remove the projection 105. Before taking such countermeasure for noise, it is necessary to perform an inspection to determine the presence or absence of a nick 103 on the gear tooth.
Over ball diameter (OBD) and runout are known as gear precision evaluation factors. As shown in FIG. 22, the OBD is defined as a dimension D measured over the outside of two balls 107 and 108 that are inserted in diametrically opposite tooth grooves of the gear 100 (for even number teeth gears, and as close as possible for odd number teeth gears). For gear teeth formed by machining, the width of the gear tooth groove becomes large as the cutting depth increases. This will allow the balls 107, 108 to move deeper in the tooth grooves, and a measured OBD becomes small. By contrast, a measured OBD becomes large as the cutting depth is reduced.
When a deviation of the measured OBD from a prescribed reference OBD value is within an allowable range, the test gear is judged as “acceptable”. By contrast, when the deviation is outside the allowable range, the test gear is judged as “unacceptable”.
It may occur that a gear is acceptable in terms of the OBD inspection but the middle point Dc of the measured ODB deviates from the center of rotation of the gear by a distance δ. The same phenomenon may also occur when the center of an axis of the gear is offset from the center of the pitch circle 102. These deviations are collectively called “runout”.
In practice, the aforementioned OBD and runout are defined as shown in TABLE 1 below.
TABLE 1NameBasic DataDefinitionOBDA plurality of OBDThe average value of the OBDmeasurement values obtainedmeasurement values is given asfrom a test gearthe OBDRunoutA plurality of OBDThe difference between ameasurement values obtainedmaximum value and a minimumfrom a test gearvalue among the measured OBDvalues is given as the runout
In the case where the nick inspection, the OBD inspection, and the runout inspection are performed separately and independently, inspection man-hour is greatly increased. To deal with this problem, a gear inspection apparatus using a master gear has been proposed as disclosed, for example, in Japanese Patent Application Laid-open Publication (JP-A) No. 10-300409.
In the disclosed gear inspection apparatus, a master gear of known precision is advanced toward until it comes into meshing engagement with a test gear which is rotatably supported at a fixed position. In this instance, zero-backlash, double-flank contact (i.e., on both left and right flanks) is established between the test gear and the master gear. The gear inspection apparatus of this type is called “double-flank composite gear inspection apparatus”. The master gear is rotated to turn the test gear more than one complete revolution and, during that time, an amount of displacement of the master gear is measured by a noncontact sensor.
The double-flank composite gear inspection apparatus disclosed in JP 10-300409A includes a frequency filter provided to eliminate a swell in the waveform before a nick inspection is performed. In this instance, because the magnitude of a nick component waveform varies with the capacity of the frequency filter used, it is necessary to select an optimum frequency filter that can eliminate the swell from the waveform within a required and sufficient range. However, selection of such optimum frequency filter is difficult to achieve and requires an increased number of man-hours.
Furthermore, it is desirable for the OBD inspection and the runout inspection to suppress the effect of a nick component waveform as much as possible. This is because such a gear, which has a nick but is acceptable in terms of the OBD and runout, can be restored into a normal acceptable gear by removing the nick with a file, for example. This means that the OBD inspection and the runout inspection require a frequency filter which can eliminate the nick component waveform and hence is different from a frequency filter used in the nick inspection. This leads to a further increase in the inspection man-hours.